Not Applicable.
Murky, turbid, water causes serious problems for underwater navigation and exploration, especially in the area of detection and removal of underwater mine hazards. In this case, it basically costs one mine detection system per mine hazard because the detection process itself detonates the mine and destroys the nearby sensor. However, if a mine could be pinpointed visually from a safe distance in murky or turbid water, it could be detonated or safely avoided without destroying the sensor. Thus, there is a need for a system that can enhance underwater visibility in turbid or murky waters or in highly scattering media, generally. In addition, the location and recovery of submerged wreckage is frequently hampered by turbid water. A system to enhance visibility in murky, turbid waters could be extremely useful in such recovery operations.
It is therefore an object of the present invention to provide a method for improving and enhancing visibility and object detection in turbid and murky water and other fluid mediums. It is also an object of the invention to provide an underwater visibility enhancement system for underwater exploration and search and rescue missions where turbid, murky, water impedes the rapid location and identification of submerged vessels and the like.
These and other objects are achieved by a system that uses an active hyperstereo imaging system to provide an observer with an improved and enhanced stereo image of the objects submersed in turbid or murky water. A matched pair of illuminating lasers and imaging cameras are alternately chopped. The left stereo camera is illuminated from the right laser and the right stereo camera is illuminated from the left laser. This produces opposite side-to-side backscatter radiation gradients from the scattering media. Specular reflection from objects and surfaces will not have this gradient so that appropriate signal processing techniques can be used to eliminate the backscatter and deblur scene content.
Another aspect of the invention of this application is to provide lasers for illumination that are optimum for looking through water. Lasers emitting light at around 532 nm or blue-green are best suited to this purpose. Another aspect of the invention of this application involves positioning specular reflectors behind the vessel since it is not practical to position specular reflectors in the forward direction. Accordingly, the vessel drags a tether behind it with a set of specular reflecting spheres. The spheres are spaced apart so as to provide sufficient spatial information to perform hyperstereo fusion and inverse filtering to minimize point spread blurring.
Another aspect of the invention of this application involves providing an illumination and camera system to utilize the aft specular reflecting spheres for deblurring. Accordingly, there are simultaneous forward and backward laser and camera pairs which are alternately chopped. Thus, an embodiment of the present application provides four instrumentation pods, each with matched lasers and cameras. Lasers in the two instrumentation pods on the same side of the vessel illuminate forward and backward while matched cameras on the other side collect both forward and backward imagery from the opposite side. The other two lasers are blocked initially and then illuminate forward and backward from the opposite side of the vessel while a second pair of matched cameras on the side of the vessel where the lasers were initially illuminating are now blocked.
In accordance with another aspect of the invention, the set of imagery from the backward looking stereo cameras is processed to reduce backscattering by using the side-to-side scattering intensity gradient caused by the off-axis illuminating lasers followed by appropriate inverse filtering to eliminate point spread function blurring. The forward looking stereo imagery has the same method for the reduction of backscatter performed followed by the deblurring inverse point spread function derived from the backward looking cameras.
Finally, in a preferred embodiment, the resultant stereo imagery is viewed by merging the two sets of stereo imagery. First, a set of backward looking image fields or frames are displayed and then a set of forward looking image fields or frames are displayed. The reference specular reflecting spheres are superimposed on the forward looking wide baseline stereo imagery. This allows the observer to maintain stereo fusion even when there are no objects, such as underwater mines, in the forward looking field of view. When there is an object such as an underwater mine in the forward looking field of view, the distance to the mine can be estimated using the stereo depth perception and determining where the mine falls in the reference grid formed by the linear array of specular reflecting spheres from the backward looking cameras which are overlaid on the forward looking imagery.
In accordance with another aspect of the invention, a pulsed laser may be used instead of a continuous wave laser is used and the opposite side cameras may be left blocked during the first few nanoseconds after the laser pulse to reduce the magnitude of the backscattered radiation from the murky water close to the vessel.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the detailed description, wherein only preferred embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive.